The present invention relates to the field of axial fans, and more particularly to an improved means for mounting fan blades on a rotatable hub.
Axial fans are widely used to transform mechanical energy, which has been supplied to them by a motor, into displacement of air. They are used wherever it is necessary to evacuate or to feed air or any other gas.
The present invention relates to an axial fan of the type comprising a hub, a plurality of arms extending radially outwardly from the hub and blade members, each blade member being secured to its respective arm.
Axial fans are generally subjected to substantial loads which act primarily on the inner ends, i.e. the roots of the blades. The loads which act on the blades are composed mainly of the traction force, acting to push the blades in the axial direction opposite to the air flow, and the ensuing component of the centrifugal force acting to push the blades back into the direction of the air flow. Consequently, some of these loads and their variations are transmitted in the form of vibrations from the blades to their roots and to the power transmission group (hub and drive), resulting in substantial wear of the fan structure. The connection between the inner ends of the blades and the hub, therefore, is critical since it is a possible source of failure due to fatigue cracking.
Currently, a reduction of the loads acting on the blades is obtained, for example, by a method which consists of mounting the blades with a fixed inclination with respect to the plane of rotation in a direction opposite to the air flow. Such an assembly can be seen in FIG. 1, showing a blade 2 connected to a hub 1 by means of an arm 3. The arm 3 is secured to the hub 1 by a clamping device 4 and to the blade 2 by a securing device 5. The blade is inclined at an angle xcex1 with respect to the plane of rotation, which is the plane perpendicular to the axis of rotation AAxe2x80x2. The main forces acting on the blade 2 are the centrifugal force CF and the traction force TF; the direction of air flow is indicated by the arrow AF.
As a result of the fixed inclination of the blades, loads can only be neutralised if the fan operates precisely in the conditions predicted by the original calculation, i.e. at a predetermined and constant speed of rotation.
Another prior art method proposes to connect the hub to the roots of the blades by a hinge, thereby enabling the blades to float when the fan is in operation. Such an assembly can be seen in FIG. 2 where, similarly to FIG. 1, there is shown a blade 2 connected to a hub 1 by means of an arm 3xe2x80x2. The arm 3xe2x80x2 is secured to the hub 1 by a clamping device 4 and to the blade 2 by a securing device 5. The main forces acting on the blade 2 are the centrifugal force CF and the traction force TF; the direction of air flow is indicated by the arrow AF. Numeral 6 designates a hinge providing a non-rigid connection between the blade 2 and the hub 1. Since both parts of the hinge are movable with respect to each other, both parts are exposed to wear and need frequent maintenance. Furthermore, the gap between the two parts of the hinge provides a space for the penetration of corrosive elements or the formation of deposits that ultimately could impede the relative movement of both parts of the hinge, preventing them from fulfilling their function of neutralising traction and centrifugal forces acting on the blades.
The object of the present invention is to provide an axial fan with a simple structure, wherein the load variations and the ensuing vibrations are substantially reduced.
In accordance with the present invention, this object is achieved by providing an axial fan of the type comprising a hub, a plurality of arms extending radially outwardly from the hub and blade members, each blade member being secured to its respective arm, characterised in that the arms consist of flexible elements with a bending stiffness such that, in operation, the blade members are inclined at an angle at which the centrifugal forces acting on the blade members neutralise the traction forces acting on same.
In an axial fan according to the present invention, the flexible elements are sized according to the operational requirements of the fan in such a way that, when the fan is in use, the blades adjust to a position where the bending moment generated by the traction forces TF is neutralised by the opposite bending moment generated by the centrifugal forces CF. Consequently, the loads acting on the roots of the blades tend to cancel each other out and, at the same time, the load variations and ensuing vibrations transmitted from the blades to the power transmission group are substantially reduced, thereby increasing the service life of the fan blades and the drive mechanism. The fan of the present invention has a higher resistance to fatigue than the fan of the prior art.
A reduction in the vibrations of the blades, as obtained by the present invention, allows the design of fan blades using less valuable materials, thinner materials, or a combination of both, which constitutes a further advantage of the invention.